Synthesis and Structural Studies of Two Paramagnetic Metallofullerenes with Isomeric C₇₂ Cage

We synthesized and isolated two paramagnetic metallofullerenes of La@C₇₂ and Y@C₇₂ with different fullerene cages, which were characterized by electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. DFT calculations disclosed two possible isomers of La/Y@C₇₂ with C₇₂-C₂ and C₇₂-C_2v cages, both of which have similar thermodynamic stability and one pair of fused pentagons. Their paramagnetic properties were then studied by EPR spectroscopy, and the obtained EPR signals were analyzed with very different hyperfine coupling constants, revealing distinct electron spin distributions for these two species. Furthermore, the experimental coupling constants were compared with those of calculated coupling constants, and comparison results revealed that the produced La@C₇₂ has a C₇₂-C_2v cage and Y@C₇₂ has a C₇₂-C₂ cage. These studies illustrate that the electron spin can be used as a probe to identify metallofullerene structure due to the susceptibility of spin-metal couplings. The successful isolation and characterizations of La@C₇₂ and Y@C₇₂ with such a small C₇₂ cage reveal their stability that is important for application as paramagnetic molecule materials.